1. Field of the Invention
The invention relates to scanning apparatus which may be used in a real-time imaging system and, in particular, in a real-time passive millimetre wave imaging system. The scanning apparatus may also be used in other radiometry systems.
2. Discussion of Prior Art
British Patent No. 700868 (February 1952xe2x80x94December 1953) describes a twistreflector which relates to a similar field as the present invention.
Millimetre wave imaging is potentially useful as an all-weather surveillance and guidance aid but any practically useful system must be capable of imaging in real-time. This is not possible using existing systems. In a millimetre wave imager, radiation from the scene to be scanned is collected by means of a concave mirror or a lens and is focused onto an array of millimetre wave receivers. At present, large two-dimensional arrays of receivers which cover the whole of a required image are not available. Instead, a far smaller number of receivers is scanned across the image in order to build up the complete picture. A similar technique is used in some infrared imagers (for example EP 0226273).
Current millimetre wave imaging systems use mechanical scanning of one or several channels to synthesise an image. Ultimately, electronic scanning and staring array techniques could be developed to implement real-time millimetre wave imaging, although there are several problems associated with such a solution. Firstly, as the wavelength is necessarily long, in order to image under adverse weather conditions the system aperture must be large to gain adequate resolution. In some millimetre wave imaging systems the input aperture may be of the order of 1 m in diameter. Secondly, the cost per channel is high so that any electronically scanned or staring array technique is expensive. Furthermore, in the case of millimetre wave staring arrays there are fundamental problems analogous to the cold shielding problems encountered in infrared systems.
Another requirement of a practical millimetre wave imaging system is that it must be able to operate at TV-compatible rates (i.e. 50 Hz for the UK, 60 Hz for the USA). In the infrared, scanning systems are often plane mirrors flapping about an axis contained within their surface. This is not a practical option in the millimetre waveband as large aperture mirrors would be required to flap back and forth at TV-compatible rates, requiring a large change in inertia at the end of each scan.
In infrared imaging systems, where input apertures are typically only 10 mm in diameter, rotary systems have been used (EP 0226273). Furthermore, in the infrared, it is usual to employ afocal telescopes to match the field of view in the scene to that of the rotating polygon. This is impractical in high resolution millimetre wave imaging where the input apertures have considerably greater diameters and afocal telescopes would need to be excessively large.
Any scanning mechanism used in a millimetre wave imaging system must therefore be situated in either the object or the image plane. Furthermore, any scanning mechanism situated in the image plane must have good off-axis performance. This is difficult to achieve using existing technology.
Another known scanning method used in infrared imagers is a system of two discs rotating about axes which are slightly inclined to the normals to their faces. Radiation incident on the first disc is reflected at oblique incidence from the first rotating disc and passes to the second disc to experience a second reflection. By varying the orientation and relative speed of rotation of the discs, varying scan patterns can be achieved. Such a two-axis rotating disc system would not be ideal for use in millimetre wave imaging, however, as the system would be inconveniently large.
It is an object of the present invention to provide a compact object space scanning apparatus which may be used, in particular, to implement real-time millimetre wave imaging, or in radar systems. It is also an object of the invention to provide a scanning apparatus which has limited power requirements and minimum inertia and gives good off axis performance.
According to the present invention, apparatus for scanning input radiation from a scene and for generating output radiation to form an image at a curved detector array comprises;
a rotatable reflective plate, forming an entrance pupil to the apparatus, for receiving and reflecting input radiation, the reflective plate having an axis of rotation passing substantially through the centre of the plate, wherein the axis of rotation is inclined at a non-zero angle xcex8a to the normal to the reflective plate at the centre of rotation,
rotary means for rotating the reflective plate and
focussing means for producing an image at the detector array
characterised in that the converging effects of the focusing means and the rotatable reflective plate co-operate with the curvature of the detector array to focus the image onto the detector array.
The rotatable reflective plate may be a substantially plane reflector.
In a preferred embodiment, the apparatus may comprise a rotatable reflective plate which exhibits a concave profile for producing an image at the detector array, thereby incorporating the function of the focusing means into the rotatable reflective plate.
In a further preferred embodiment a dielectric layer is located on the surface of the concave rotatable reflective plate to form a rotatable Mangin mirror configuration, for providing optical correction. The profile of the dielectric layer may be different to the profile of the rotatable reflective plate. The surface of the dielectric layer and the surface of the rotatable reflective plate are preferably both aspheric.
In particular, the apparatus is such that astigmatism and coma aberrations in the image formed at the detector array are minimised.
In a further preferred embodiment the rotatable Mangin mirror may be used in conjunction with
a lens arrangement located between the rotatable reflective plate and the detector array arranged to receive both input radiation from the scene and input radiation reflected from the rotatable reflective plate, and being capable of selectively transmitting and focusing radiation having a particular direction of polarisation, the lens arrangement comprising a first polarising element, for selectively transmitting and selectively reflecting radiation having a particular direction of polarisation, the first polarising element having a first polarisation axis and having a substantially flat surface, a second element for rotating the direction of polarisation of radiation through substantially 45xc2x0 and a third polarising element, for selectively transmitting and selectively reflecting radiation having a particular direction of polarisation, the third polarising element having a third polarisation axis and having a substantially flat surface, the third polarisation axis making an angle of substantially 45xc2x0 with the first polarisation axis,
The arrangement of the apparatus being arranged such that the optical path within the apparatus is reduced and a compact configuration is achieved.
In a further preferred embodiment, apparatus for scanning input radiation from a scene and for generating output radiation to form an image at a detector array comprises;
a rotatable reflective plate, forming an entrance pupil to the apparatus, for receiving and reflecting input radiation, the reflective plate having an axis of rotation passing substantially through the centre of the plate, wherein the axis of rotation is inclined at a non-zero angle xcex8a to the normal to the reflective plate,
rotary means for rotating the reflective plate and
a lens arrangement arranged to receive both input radiation from the scene and input radiation reflected from the rotatable reflective plate, and being capable of selectively transmitting and focusing radiation having a particular direction of polarisation, the lens arrangement comprising a first polarising element, for selectively transmitting and selectively reflecting radiation having a particular direction of polarisation, the first polarising element having a first polarisation axis and having a substantially flat surface, a second element for rotating the direction of polarisation of radiation through substantially 45xc2x0 and a third polarising element, for selectively transmitting and selectively reflecting radiation having a particular direction of polarisation, the third polarising element having a third polarisation axis and a substantially spherical surface having a centre of curvature, C, and a radius of curvature, R, the third polarisation axis making an angle of substantially 45xc2x0 with the first polarisation axis,
wherein (i) the axis of rotation of the plate passes through the centre of curvature, C, and
(ii) the separation of the first polarising element and the centre of curvature, C, along the axis of rotation and the separation between the first polarising element and the plate along the axis of rotation are substantially equal,
the arrangement of the apparatus being such that in use optical aberrations in the image formed at the detector array are minimised.
In particular, the apparatus is such that astigmatism and coma aberrations in the image formed at the detector array are minimised.
The apparatus may also comprise a feed horn array comprising a plurality of feed horns, the feed horns forming part of a spherical surface having a radius of curvature substantially equal to R/2 and being concentric with the third polarising element. The apparatus may also comprise a detector array, comprising a plurality of detector elements. The detector array may form part of a millimetre wavelength imaging camera.
Preferably, the first polarising element may be a planar grid of wires and the third polarising element may a substantially spherical grid of wires.
The apparatus may comprise two or more lens arrangements arranged in series.
In a further preferred embodiment, the apparatus may further comprise a corrector plate located between the rotatable disc and the third polarising element for removing spherical aberrations from an image formed at the detector array.
In a further preferred embodiment, the corrector plate is a weak converging element to allow the over-dimensioning of the lens arrangement to be eliminated.
In a further preferred embodiment the corrector plate exhibits an aspheric convex profile.
In a further preferred embodiment, the rotatable reflective plate is a weak converging element for removing spherical aberrations from the image formed at the detector array and to allow the over-dimensioning of the lens arrangement to be eliminated.
In a further preferred embodiment the rotatable reflective plate is a strong converging element to allow the converging power of the lens arrangement to be reduced.
In a further preferred embodiment the rotatable reflective plate exhibits an aspheric concave profile.
The apparatus may also include a radar receiver.
In a further preferred embodiment, the angle of inclination xcex8a is between 1xc2x0 and 10xc2x0.
For the purpose of this description, the term xe2x80x9cpolarising elementxe2x80x9d shall be taken to mean any element which acts on a particular polarisation of radiation, including elements which act on unpolarised radiation but which do not produce polarised radiation.